1. Field of the Invention
The present invention relates to a communication apparatus, a communication method, and a computer program, and in particular, relates to a communication apparatus that performs communication by the MIMO (Multiple Input Multiple Output) method, a communication method, and a computer program.
2. Description of the Related Art
There is a radio communication system that increases a transmission capacity by providing a plurality of antennas on each of transmitting and receiving sides and by performing communication using space division multiplexing (MIMO method: Multiple Input Multiple Output) with the plurality of antennas.
Among MIMO receiving methods, the maximum likelihood detection (MLD) that shows the best characteristics has proved extremely resistant to various error components (such as phase noise, amplitude distortion, frequency offset, and clock shifts) due to imperfection of RF (Radio Frequency).
FIG. 5 is an explanatory view showing PER (Packet Error Rate) characteristics of the MMSE (Minimum Mean Squared Error) method and the MLD method as a graph. The modulation method in FIG. 5 is an MCS (Modulation and Coding Scheme) 12 of 64 QAM (Quadrature Amplitude Modulation)×2. As shown in FIG. 5, characteristics of MLD are worse than those of MMSE only if a frequency offset is applied even after a correction using pilot information inserted in an 11n format of WLAN (Wireless LAN) is made.
Investigation of resistance to phase errors based on simulation evaluation revealed that it is necessary to control a phase error to about 1° to 2° when 64 QAM×2 is used to maintain original characteristics of MLD, which shows that extremely high accuracy is demanded from 64 QAM. When only a phase error is considered, even if 3% RMS (30 [dB]) of EVM (Error Vector Magnitude) of RF is achieved, potentially about 3° of a nonlinear phase error will typically be present. In WLAN, Control Channel is not constantly transmitted like cellular communication and therefore, it is very difficult to prevent characteristic degradation due to reception of MIMO-MLD of 64 QAM.